Laser system using phase change material for thermal control

ABSTRACT

A passively cooled solid-state laser system for producing high-output power is set forth. The system includes an optics bench assembly containing a laser head assembly which generates a high-power laser beam. A laser medium heat sink assembly is positioned in thermal communication with the laser medium for conductively dissipating waste heat and controlling the temperature of the laser medium. A diode array heat sink assembly is positioned in thermal communication with the laser diode array assembly for conductively dissipating waste heat and controlling the temperature of the laser diode array assembly. The heat sink assemblies include heat exchangers with extending surfaces in intimate contact with phase change material. When the laser system is operating, the phase change material transitions from solid to liquid phase. This transition of the phase change material also provides a thermal buffer for laser components such that the phase change material absorbs the energy associated with fluctuations in ambient temperature before transferring it to the laser component. Also, the heat sink assembly can contain more than one type of phase change material, each having a different melting temperature.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 09/270,991,filed Mar. 17, 1999, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/151,851, filed Sep. 11, 1998.

FIELD OF THE INVENTION

The present invention relates generally to a solid-state laser systemand, in particular, to a solid-state laser that is passively cooled andthermally controlled by heat sink bodies containing phase changematerial.

BACKGROUND OF THE INVENTION

Solid-state laser systems are characterized in that they have asolid-state laser gain medium which converts energy from an optical pumpsource to a coherent output laser beam. The pump source can be one ofmany available energy-producing systems such as flash lamps orsemiconductor laser diodes. The energy produced by the pump source isincident upon the laser medium and absorbed by the laser medium.

The absorbed energy in the laser medium causes the atoms in the lasermedium to be excited and placed in a higher energy state. Once at thishigher state, the laser medium releases its own energy which is placedinto an oscillating state by the use of a laser resonator. The laserresonator includes at least two reflective surfaces located on eitherside of the laser medium. The laser resonator can be designed tocontinuously release a laser beam from the system. Alternatively, theresonator can be designed such that when the energy oscillating throughthe laser medium reaches a predetermined level, it is released from thesystem as a high-power, short-duration laser beam. The emitted lightproduced from the solid-state laser system is generally coherent andexits the system in a predefined area.

In many systems, the laser medium is Neodymium-doped, Yttrium-AluminumGarnet (Nd:YAG). A laser medium made from Nd:YAG absorbs optical energymost readily when the energy is at a wavelength of approximately 808nanometers (nm). Thus, the source to pump the Nd:YAG laser medium shouldbe emitting light energy at approximately 808 nm. Gallium arsenidesemiconductor laser diodes can be manufactured with dopants (e.g.aluminum) that will cause the emitted light to be in a variety ofwavelengths, including 808 nm. Thus, the semiconductor laser diodes,which are lasers by themselves, act as the pump source for the lasermedium.

The conversion of optical energy into coherent optical radiation isaccompanied by the generation of heat which must be removed from thedevice. Cooling of the laser medium reduces the build-up of temperaturegradients and, thereby, the strain and stress in the laser medium andalso avoids the likelihood of laser medium fracture due to highthermo-elastic stress. Also, variation of the refractive index and itsassociated optical distortion can be largely controlled or avoided byeffective cooling. The result is improved beam quality and/or increasedaverage output power.

Diode array performance is also strongly dependent on temperature. Notonly is the output power a function of temperature, but the wavelengthof the emitted energy that is to be absorbed by the laser medium is alsoa function of diode temperature. To maintain desired array performanceand to prevent the diode array from being destroyed by overheating,cooling of the area surrounding the array is also important.

Other laser assembly components, some having low damage thresholds, alsorequire close temperature control. For example, beam dumps, that absorband dissipate incident laser energy to ensure that incident laser energywill not emerge to interfere with wanted parts of the beam, produceheat. Nonlinear crystal assemblies for the conversion of wavelengths ina laser system utilize temperature control systems for the precisecontrol of these temperature-sensitive crystals. Careful attention isalso given to the optimal transfer of heat from acousto-opticQ-switches.

It has been an objective for laser manufacturers to develop high-power,solid-state systems. As the output power in these system increases, thewaste heat increases which puts more demands on cooling systems andnecessitates larger volumes in which to provide adequate cooling. Hence,the efficient and effective removal of waste heat from diode arrays, thelaser medium, and other heat-generating components is an importantfactor in developing compact, high-powered laser systems.

Known laser systems utilize active cooling. Active cooling systems mayuse thermoelectric coolers, or fluid systems having mechanical pumps andcoolant carrying tubing operated at pressure. However, active coolingsystems consume additional power to control the temperature of the laserand require additional space in the laser system. Furthermore, activecooling requires feedback control systems to adjust the amount ofcooling that is necessary to maintain the laser components at theappropriate temperature.

SUMMARY OF THE INVENTION

The present invention is a passively cooled, diode-pumped solid-statelaser system producing a high-power laser beam. The system includes atleast one diode array producing optical energy that is absorbed by asolid-state laser medium. The solid-state laser medium has an outersurface into which optical energy from the diode array is emitted.

The laser system further includes a pair of opposing reflective surfacessubstantially optically aligned along a central axis of the laser mediumand positioned with the laser medium therebetween. One of the opposingreflective surfaces is an output coupling mirror for reflecting aportion of energy produced by the laser medium to provide laserresonation and also for transmitting the high-power laser beam.

To provide the passive cooling of the laser medium, a laser medium heatsink assembly contains a substantially solid form of phase changematerial in thermal communication with the laser medium. The solid formof the phase change material changes to a liquid form of the phasechange material in response to heat from the laser medium beingtransferred to the laser medium heat sink assembly.

To absorb the heat from the diode array, a diode array heat sinkassembly contains a substantially solid form of phase change material inthermal communication with the diode array. The solid form of the phasechange material changes to a liquid form of the phase change material inresponse to heat from the diode array being transferred to the diodearray heat sink assembly.

While the laser system cannot be operated endlessly with only passivecooling, passive cooling can provide the necessary cooling for a lasersystem for several minutes. Such a system can be useful in manyapplications such as the terminal guidance system for a missile.Advantages to be gained from passive cooling include more compact,portable, lighter, and vibration free laser systems. Additionally, alaser system with more effective passive cooling can accommodate theincreased heat transfer associated with a more powerful laser.

Furthermore, employing a phase change material in combination with theheat exchanger having a working medium flowing therethrough providestemperature control of laser components in addition to heat absorptionproperties. Thermal control is provided by the latent heat associatedwith the phase change material. A material in its solid phase willcontinue to absorb energy and remain at a constant temperature (itsmelting point) until a specified amount of energy is absorbed completingthe transition from solid to liquid phase. Furthermore, an interface inintimate contact with the phase change material proceeding through thistransition will be held at approximately a constant temperature untilthe transition from solid to liquid is complete.

To provide for more continuous operation of the laser system using aphase change material, the heat sink assembly containing phase changematerial is placed in thermal communication with a heat exchangercontaining working fluid. The liquid form of the phase change materialchanges to a solid form in response to heat being transferred from theheat sink assembly to the heat exchanger. Also, the heat exchanger canbe operated in reverse (i.e. transfer heat from the working fluid, or aheater, to the phase change material) to liquefy the phase changematerial and, thereby, maintain temperature-sensitive components atoptimal operating temperatures.

The heat sink assembly containing phase change material provides athermal buffer for laser components when the ultimate heat sink, such asthe ambient air, is subject to temperature fluctuations. The thermalbuffer is associated with the latent heat of fusion of the phase changematerial as it undergoes a phase change. The temperature of the lasercomponent generally remains constant as the energy associated withchanges in ambient temperature is absorbed in the phase change materialbefore it is transferred to the laser component. The thermal controlprovided by the phase change material alleviates the need for anelectronic thermal-control loop.

Additional thermal control qualities are provided by another embodimentin which a heat sink assembly containing phase change material is placedin thermal communication with a thermoelectric cooler. With thethermoelectric cooler disposed between the temperature-sensitivecomponent and the heat sink heat is transferred from the component,across the thermoelectric cooler, and into the heat sink. With the heatsink assembly disposed between the temperature-sensitive component andthe thermoelectric-cooler, the phase change material is maintained inits melt phase as heat is removed from the phase change material by thethermoelectric cooler. Also, the thermoelectric cooler can be operatedto discharge heat into the heat sink assembly if it is desired to raisethe temperature of any system component.

In another embodiment, the heat sink contains more than one type ofphase change material, each having a different melting temperature. Inthis embodiment, the thermal gradient can be tailored, for example, byplacing phase change material with a greater melting temperature incavities closer to the temperature-sensitive component relative tocavities filled with a phase change material having a lower meltingtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 is a perspective view of the solid-state laser system of thepresent invention;

FIG. 2 is a side-elevational, cross-sectional view along 2—2 of FIG. 1of the solid-state laser system of the present invention;

FIG. 3 is a top view of the solid-state laser system of the presentinvention;

FIG. 4 is an exploded view of the laser medium heat sink assembly, diodearray assembly, laser medium, and diode array heat sink assembly of thepresent invention;

FIG. 5 is an exploded view of the laser medium heat sink assembly andlaser medium of the present invention;

FIG. 6 is a front cross-sectional view along 6—6 of FIG. 4 of the lasermedium heat sink assembly and laser medium;

FIG. 7 is a front cross-sectional view along 7—7 of FIG. 4 showing thelaser medium heat sink assembly, laser medium, diode array, and diodearray heat sink assembly;

FIG. 8 is a plot of the output of the laser system versus time whenoperated at an input current of 45 A, repetition rate of 500 Hz, andpulsewidth of 200 μsec;

FIG. 9 is a plot of output power wavelength versus time for test runs atpeak input currents of 45, 50, 55, and 60 A;

FIG. 10 is a cross-sectional view of the heat sink assembly, heatexchanger, and laser diode array;

FIG. 11 is a cross-sectional view of another embodiment showing the heatsink assembly, heat exchanger, and laser diode array;

FIG. 12 is a cross-sectional view of another embodiment showing the heatsink assembly, heat exchanger, and laser diode array;

FIG. 13 is a side view, partially schematic, illustrating the diodearray and laser medium in thermal communication with two dual-stagetemperature control systems;

FIG. 14 is cross-sectional view of the laser diode array, heat sinkassembly, thermoelectric cooling device, and actively cooled heatexchanger;

FIG. 15 is a cross-sectional view of the laser diode array,thermoelectric cooling device, and heat sink assembly;

FIG. 16 is a cross-sectional view of a heat-generating component, heatsink assembly, heater, and floor of the optics bench assembly; and

FIG. 17 is a cross-sectional view of the laser diode array and heat sinkassembly having multiple types of phase change material.

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed. Quite to the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 and FIG. 2, a solid-state laser system 10 forproducing a high-power laser beam 11 is illustrated. The laser system 10includes an optics bench assembly 12 that is the mounting structure forvarious optical components and a laser head assembly 14 which generatesthe high-power laser beam 11.

The optics bench assembly 12 includes the optical components (discussedbelow) and a housing unit 16. The housing unit 16 is a rectangular blockof material (e.g. brass) with its center removed. The housing unit 16includes a floor 18, a first end piece 20, a second end piece 22, afirst sidewall 24, a second sidewall 26, and a bottom cover 28. Mounts30 are integrally formed in the housing unit 16 to secure the lasersystem 10 into a larger assembly.

With particular reference to FIG. 2, the first end piece 20 includes abeam output window 32 for the exiting of the laser beam 11. The secondend piece 22 includes an alignment window 38 which is centered on theaxis of the laser beam 11. The alignment window 38 is covered byremovable opaque plug. When the plug is removed, a low-power, eye-safelaser beam (e.g. a He—Ne laser) from an external source can be sentthrough this window 38 to determine where the exact location of thelaser beam 11 will be when the laser system 10 is operated. Thus, theoperator of the laser system 10 is not required to align the beam withthe optical components.

To provide electrical connection for the laser system 10, the firstsidewall 24 includes an electrical port which provides access for thewires conducting the electrical energy to the laser system 10. Wiressimply pass from the internal components within the housing 16 to aconnector assembly 40 located within the port. An external electricaldrive and control system would then be coupled to the connector assembly40.

As best seen in FIG. 2, the floor 18 of the housing unit 16 has severalbores 42 for mounting various components. Some of these bores 42 may bethreaded while some may simply act as through-bores for receivingfasteners from the underside of the optics bench assembly 12 thatthreadably engage threaded bores on the optical components.

FIGS. 1, 2, and 3 also illustrate the optical components utilized in onepreferred operational system that provides a pulsed mode of operation.These components include an output coupling (OC) mirror assembly 44, apolarizer cube 48, an electro-optic Q-switch 50, a waveplate 52, aRisley prism pair 54, and a highly-reflective (HR) mirror assembly 56.Additionally, an aperture assembly 36 is positioned adjacent to the OCmirror assembly 44. Thus, when the laser head assembly 14 converts theelectrical energy into optical energy, these optical components act uponthat optical energy to produce the resultant laser beam 11.

Focusing now on FIG. 2, the laser head assembly 14 includes a lasermedium 58, a laser medium heat sink assembly 60, laser diode arrays 62,and a diode array heat sink assembly 64. The laser medium 58 is disposedbetween the laser medium heat sink assembly 60 and the diode arrays 62that are adjacent to the diode array heat sink assembly 64. In operationthe diode arrays 62 emit energy at a first wavelength that is absorbedby the laser medium 58 and converted to energy at a second wavelengthresulting in laser beam 11.

Each laser diode array 62 includes a plurality of laser diode bars whichconvert electrical energy into optical energy. Six diode arrays 62 areshown in FIG. 4. To improve the thermal efficiency of the entire system,each laser diode array 62 is soldered to the diode array heat sink 64.The laser diode arrays 62 are usually of the type having anon-electrically conductive lower substrate (e.g. Beryllium Oxide) asshown, for example, in U.S. Pat. No. 5,128,951 to Karpinski which isherein incorporated by reference in its entirety. The laser diode arrays62 are electronically connected in series with each other. Consequently,there is one electrical input wire connected to an input contact (solderpad) 66 and one electrical output wire connected to an output contact(solder pad) 68 for all of the laser diode arrays 62.

As mentioned above, the optical energy from the laser diode arrays 62 isabsorbed by the laser medium 58. The amount of absorption of energy bythe laser medium 58 at a given wavelength depends on various factorssuch as the type of dopants provided in the laser medium 58, theconcentration of dopants, and the temperature at which the laser medium58 is operated.

In one preferred embodiment, if the laser medium 58 is made fromNeodymium (3+) doped, Yttrium-Aluminum Garnet (Nd:YAG), the peakabsorption occurs at about 808 nm. Also, other laser mediums such asNd:YLF can be used. When the laser diodes from the laser diode arrays 62are made of gallium arsenide with aluminum doping (AlGaAs), they emitradiation at approximately 808 nm which matches the maximum absorptionspectrum for the Nd:YAG material. When the laser medium heat sink 60 isapproximately 30-40° C., the Nd:YAG laser medium in direct contact withthe laser medium heat sink 60 absorbs the 808 nm energy well. When anNd:YAG laser medium absorbs energy at 808 nm, it then releases energy ata wavelength of about 1064 nm that results in laser beam 11.

Still referencing FIGS. 1-3, to produce laser resonation, a reflectivesurface is positioned outside of each end of the laser medium 58 tocause energy to be continuously sent through the laser medium 58. At oneend, the HR mirror assembly 56 is positioned adjacent to the second endpiece 22 of the optics bench 12 and connected thereto with fasteners.The HR mirror assembly 56 includes a high-reflective (HR) mirror 74 witha front surface that has a reflectivity value of at least about 99% whenthe wavelength is 1064 nm. Also, the mirror 74 transmits energy at otherwavelengths such that an alignment beam that is sent through thealignment window 38 is transmitted through the HR mirror 60 and intoother optical components.

At the other end, an output coupling (OC) mirror assembly 44 ispositioned adjacent to the first end piece 20 of the optics bench 12 andconnected thereto with fasteners. The OC mirror 78 has a partiallyreflective coating on its surface such that a predetermined amount ofenergy is transmitted therethrough and released through the beam outputwindow 32 as the laser beam 11. The remaining energy is reflected backthrough the optical components. The reflectivity of the OC mirror 78determines the overall output in the laser beam 11. Also, thereflectivity must be enough to produce resonation through the lasermedium 58. The OC mirror 78 can have a reflectivity that ranges fromabout 5% to about 94% (i.e. about 95% to 6% is transmitted as laser beam11) with the optimum value being dependent on the application. In apreferred embodiment, the reflectivity of the OC mirror 78 is about 90%for a laser system 10 operating in a CW mode. For a laser systemoperating in a pulsed mode, the reflectivity of the OC mirror 153 isapproximately 70%. An OC mirror with a reflectivity of about 80% wouldserve both modes of operation.

In a preferred embodiment, the polarizer cube 48 is positioned adjacentto the laser head assembly 14 and is pivotally mounted to the floor 18of the optics bench 12. The cube 48 includes two joined prisms withalternating layers of material having high and low indices of refractionfor effecting a polarization split of the laser beam 11.

If the laser system 10 is to provide a pulsed output, the electro-opticQ-switch 50 is disposed between the polarizer cube 48 and the waveplate52, aligned with the central axis of the laser medium 58 and mounted tothe floor 18 of the optics bench 12 with fasteners. When the Q-switch 50“opens” to allow for optical transmission, energy can resonate betweenthe two reflective surfaces such that a high-energy, short-durationpulse exits from the system 10. It should be noted that the Q-switch 50can be placed on either side of the laser medium 58 and that other typesof Q-switches, such as an acousto-optic Q-switch or passive Q-switch,can be used.

Further adjustment of the laser beam 11 is provided by the waveplate 52and Risley prism pair 54. The waveplate 52 is positioned between theQ-switch 50 and the Risley prism pair 54 and is connected to the opticsbench 12 with fasteners. The Risley prism pair 54 is positioned betweenthe waveplate 52 and HR mirror assembly 56 and includes two prisms 80that are rotatably mounted to the floor 18 of the optics bench 12. TheRisley prism pair 54 is used to substantially linearly deflect a beam ofwave energy. The prisms 80 can be rotated to effectuate maximumresonation of beam energy along the central axis of the laser medium 58.The waveplate rotates the polarization state of the laser beam 11 toallow proper Q-switch operation.

The laser system 10 may require a specific internal environment foroptimum operation. For example, a cover can completely enclose and sealthe system 10 which then could be back-filled with dry nitrogen if it isequipped with a simple valve on its external surface. Alternatively, thefinal assembly step could be performed in a low-moisture atmosphere. Inyet a further alternative, the laser system 10 may include a desiccantwithin the housing 14 that absorbs the moisture once a cover is sealedin place.

To provide passive cooling, the laser diodes 62 and laser medium 58 areheat sunk to unique heat exchangers having phase change materials. Thesecomponents are illustrated in FIGS. 4-7 and will now be described.

Referring now FIGS. 4-7, exploded and cross-sectional views of the lasermedium heat sink assembly 60, the diode array heat sink assembly 64 andlaser medium 58 are shown. The laser medium heat sink assembly 60includes a laser medium heat exchanger 84 with a base plate 86 having aplurality of fins 88 and a housing 90 for enclosing the heat exchanger84. The laser medium heat exchanger 84 can be made from anyhighly-conductive and preferably light-weight material including metals,metal composites, and highly-conductive non-metals.

In a preferred embodiment, the fins 88 are substantially rectangular inshape, extend along the length of the laser medium 58, and are disposedparallel with respect to each other to form interstices 126therebetween. The fins 88 may have a variety of shapes and are notlimited to the substantially rectangular fins 88 shown in FIG. 5. Othervariations that produce heat-conducting extended surfaces include tubefins, spines, grooves, plate fins of other shapes, plate baffleconstructions, internal fin-tube constructions, and a shell-and-tubeconstruction. While the fins 88 are copper and shown as parallel, theycan be made from any highly-conductive metal or non-metal and have aradial configuration as is shown with respect to the laser diode arrayheat sink 64.

The housing 90 includes a body 92 and a cover 94. The body 92 is formedby machining a substantially rectangular block of material (e.g. brassor copper) to remove its center portion leaving a substantiallyrectangular collar with a first sidewall 96, a second sidewall 98, afirst end wall 100, and a second end wall 102. The inner surfaces of thewalls 96, 98, 100, 102 are fairly smooth. The body 92 has an integrallyformed upper lip 104 at the upper portion of the walls 96, 98, 100, 102and a lower lip 106 at the lower portion of the walls 96, 98, 100, 102.The upper and lower lips 104, 106 extend outwardly from the walls 96,98, 100, 102 and are interconnected by integrally formed pillars 108having bores 110 machined therein for accepting fasteners. The housing90 can be made from materials other than copper or brass and,preferably, from materials that are non-corrosive and lightweight.

A plurality of apertures 112 are formed in the upper lip 104 to be inpositional agreement with a plurality of apertures 114 formed in thelower lip 106. The apertures 112 and 114 are also in registry withapertures 116 in the base plate 86 of the laser medium heat exchanger84. The lower portion of the body 92 has an integrally formed channel118 (FIG. 6) for receiving an O-ring 120 to prevent leaks. The body 92is secured to the base plate 86 of the laser medium heat exchanger 84with fasteners passed through apertures 114 in the lower lip 106 andapertures 116 in the base plate 86 sandwiching the O-ring 120 betweenthe base plate 86 and the body 92. The upper portion of the body 92 hasa similar channel for accepting an O-ring.

Once the body 92 is mounted to the base plate 86 of the heat exchanger84, phase change material (PCM) 122 is added within a chamber 124defined by the inner surfaces of the walls 96, 98, 100, 102 of the body92, and the interstices 126 of the fins 88. The cover 94 is then addedto the assembly which is a substantially rectangular plate havingapertures 128 in positional agreement with the apertures 112 of theupper lip 104 for accepting fasteners. The cover 94 seals the chamber124. In an alternative embodiment, the housing 90 can constitute aunitary body.

As shown in FIG. 4, six diode arrays 62 are disposed adjacent to a lowerface 70 of the laser medium 58. The lower face 70, where the energy fromthe laser diode arrays 62 enters the surface of the laser medium 58, iscovered with a coating that allows external transmission of 808 nmradiation but is internally reflective of 1064 nm radiation. An upperface 72 of the laser medium is covered with a coating reflective of both1064 and 808 nm radiation. One example of such a coating is 2000Angstroms of silver which is deposited on the laser medium 58 with avacuum-evaporation process. Thus, optical energy from the diode arrays62 enters the laser medium 58 at the lower face 70, travels through thelaser medium 58, bounces off the internally reflective coating on theupper surface 72 and is transmitted back through the laser medium 58.This path is sufficiently long for the laser medium 58 to absorb most ofthe energy from the laser diode arrays 62. Any heat produced in thelaser medium 58 is conducted into the laser medium heat exchanger 84.

To efficiently conduct heat from the laser medium 58 to the laser mediumheat sink assembly 60, the laser medium 58 preferably is attached to thebase plate 86 with highly conductive material. A preferred embodimentinvolves attaching the laser medium 58 directly to the laser medium heatsink assembly 60 with a thermally conductive adhesive such as athermally conductive room temperature vulcanization (RTV) epoxy.

Referring now to FIG. 7 and with particular reference to FIG. 4,cross-sectional and exploded views of the laser head assembly 14 areshown. The diode array heat sink 64 includes a diode array heatexchanger 130 with a base plate 132 having a plurality of fins 134 and ahousing 136 for enclosing the heat exchanger 130.

In a preferred embodiment, the fins 134 are branched and extend radiallyfrom the base plate 132 along the length of the laser medium 58. Theextended surfaces may have a variety of shapes and are not limited tothe radially branched fins shown in FIG. 4. Other variations can includetube fins, spines, grooves, plate fins of other shapes, plate baffleconstructions, internal fin-tube constructions, and a shell-and-tubeconstruction.

The housing 136 includes a body 138 having a semi-cylindrical surface140, a first end wall 142, a second end wall 144, and an access cover146 defining a chamber 148 in which a phase change material is placed.At an upper end, the body 138 has a lip 150 integrally formed therewith.Apertures 152 formed in the lip 150 accept fasteners and are in registrywith apertures 154 of the base plate 132. A channel 156 for accepting anO-ring 158 is also integrally formed in the body 138 at the upper end.

Because the heat exchanger 130 is filled with phase change material, thefirst end wall 142 has a hole 164 for providing access to the chamber148 within the body 138. The access cover 146 includes apertures 166 andan integrally formed channel for accepting an O-ring 168 to providesealing engagement with the first end wall 142. The access cover 146 issecured to the first end wall 142 with fasteners.

When the base plate 132 is mounted on the lip 150, a sealing engagementis formed with the O-ring 158 positioned within the channel 156. Withthe apertures 154 of the base plate 132 in alignment with the apertures152 in the lip 150, fasteners are passed therethrough to securely mountthe diode array heat exchanger 130.

To mount the diode array heat sink 64 to the optics bench 12, fastenersare passed from the optics bench 12 to bores 162 in mounting pillars160.

The diode arrays 62 are directly contacting the base plate 132 of theheat exchanger 130 to thermally conduct heat away from the diode arrays62 and into the diode array heat sink 64. Thus, heat produced by thediodes is transferred into the heat sink 64 where it is ultimatelyabsorbed by the PCM.

To place the laser medium 58 directly over the laser diode arrays 62,brackets 172 position and secure the laser medium heat sink assembly 60to the base plate 132 of the diode array heat exchanger 130. Eachbracket 172 has a plate 174 with an integrally formed flange 176. Theplate 174 has two slots 178 aligned with bores 110 in the heat sink body92 for passing fasteners therethrough. The flange 176 of the bracket 172has apertures 182 for securing the bracket 172 to the base plate 132 ofthe diode array heat exchanger 130.

Because of the desire to reduce the weight of the overall system,additional material is machined from the various components in areaswhere the structural integrity of the system 10 is not compromised. Forexample, a recess 180 is also formed in the plate 174 for reducing theweight of the unit.

The phase change material (PCM) 122 placed within the chamber 124 of thelaser medium heat sink assembly 60 and the PCM 170 placed into thechamber 148 of the diode array heat sink 64 change from solid to liquidat a desired temperature depending upon the demands of a particularapplication. Selecting as a working medium a PCM that transitions fromsolid to liquid as opposed to liquid to gas is advantageous in that thePCM dissipates waste heat by conduction as opposed to conduction andconvection. Also, the PCM provides thermal control of elements inthermal communication with the PCM. Thermal control is provided by thePCM's latent heat associated with the phase change. A PCM in its solidphase will continue to absorb energy and remain in its “melt phase” at aknown temperature until a specified amount of heat is absorbed tocomplete the entire transition from solid to liquid phase. Thus, anyelement in intimate contact with the PCM undergoing a phase change willbe held at a generally constant temperature that coincides with thePCM's melting temperature until the phase change is complete.

The duration of the phase change associated with a particular amount ofPCM affects the time period for operating the laser system beforereaching catastrophic temperature levels. Selecting a PCM requiresconsideration of factors other than the desired control temperature andoperation period associated with the particular laser application anddesign. One factor is the ambient temperature of the environment inwhich the laser system 10 is to operate. A PCM is selected that has amelting point above the maximum ambient temperature of the environmentin which the laser system 10 resides so that the PCM will remain in itssolid phase before laser operation begins. This temperature ispreferably in the range of −35 to 55° C. Other factors include thedesired laser power output, size of both the laser medium and the laserdiode array, and the efficiency of the laser diodes and laser mediumwhich is proportional to the waste heat.

In a preferred embodiment, gallium is selected as the PCM to serve asthe working medium. Gallium has a melting point of 29.8° C. and a latentheat of fusion of 80 J/g. The melting point of gallium closelycorresponds to an acceptable operational temperature (30°C.) of theNd:YAG material of the laser medium in the preferred embodiment. Sinceit is possible for a PCM to be a solid at room temperature but a liquidslightly above room temperature, integrating the PCM into a heatexchanger is fairly easy. Other possible PCMs include alkylhydrocarbons, salt hydrates, and low temperature metallic alloys(fusible alloys).

However, gallium, even when in its liquid phase, does not easily wet tocopper or other materials from which the heat exchanger may beconstructed. One method for integrating the PCM into a heat exchangerincludes heating the heat exchanger to a temperature above the liquidphase of the PCM. This step makes it easier to maintain the PCM in itsliquid phase while it is poured into the heat exchanger. The next stepinvolves heating the PCM until it melts to facilitate the transfer ofPCM into the heat exchanger. Next, the heat exchanger is coated with ahighly active organic fluxing agent such as Flux No. 4 by the IndiumCorporation of America of Utica, N.Y. which helps the PCM wet onto thesurface of the heat exchanger. Then, the PCM is injected or poured intothe heat exchanger. Finally, excess fluxing agent is removed. The lasttwo steps may be performed simultaneously.

The laser system 10 including a slab-shaped Nd:YAG laser medium 58having dimensions of 3.1 mm (thickness) by 6.2 mm (width) by 83.3 mm(distance tip-to-tip) has been tested. This slab was bonded to agallium-filled heat sink with thermally conductive RTV. The laser mediumheat exchanger 84 with fins 88 was machined from copper and the chamber124 had a gallium PCM volume of 0.26 in³.

Six diode arrays each having 15 diode bars were soldered to the diodearray heat sink. The diode array heat exchanger was also machined fromcopper having radially extending fins that circumscribe a semi-circlehaving a radius of 0.82 in. The chamber 148 of the diode array heat sinkhaving a PCM volume of 1.2 in³ was filled with gallium.

Referring now to FIG. 8, there is shown a plot of the output of thelaser system 10 versus time when the system was operated at anelectrical input of 45 A, a repetition rate of 500 Hz, a currentpulsewidth of 200 μsec and the physical conditions described in theprevious paragraph. For a maximum energy output of about 60 mJ, themaximum laser output power is 30 W of 1064 nm energy. The correspondingheat load produced by the slab was calculated to be 83 W and the heatload produced by the diode arrays was calculated to be 520 W. If thepower output of the entire system is desired to be less than 30 W, thenthe time of temperature-controlled operation of the slab and arrays willincrease proportionally.

Referring now to FIG. 9, there is shown a plot of the output powerwavelength versus time for test runs at peak input currents of 45, 50,55, and 60 A, a 250 μsec pulsewidth, and repetition rate of 500 Hz butunder different physical conditions than described above. The physicalconditions included only one diode subarray as opposed to the sixpreviously described. Furthermore, a slightly larger diode array heatexchanger was used. The heat exchanger had twice the effectivecross-sectional area for heat dissipation and the fins circumscribed asemi-circle having a radius of 1.16 in instead of 0.82 in previouslydescribed. Since the amount of heat dissipation is directly proportionalto the effective cross-sectional area, the amount of heat dissipationcan be easily calculated if more diode subarrays are added. As areference point, at 60 A, the waste heat of the diode arrays is about140W.

Aluminum doped gallium-arsenide (AlGaAs) diodes shift wavelength by onenanometer for approximately 4° C. change in temperature. For example,over a time period of approximately 60 seconds at an input current of 60A, the corresponding temperature change of approximately 32° C. wasmeasured (814 nm-806 nm). However, at an input of 60 A and afterapproximately 3 seconds, the wavelength remains relatively stable forapproximately 50 seconds (809 nm to about 812 nm). This flattening outof the curves is associated with the latent heat of fusion of gallium.After about 50 seconds, the rate of the change in wavelength is shown tobegin to increase. This change corresponds with the point in time whengallium has completely melted after which gallium behaves as a normalsuperheated liquid.

In addition, stress tests to verify the survivability of the lasermedium slab were conducted at various heat loading levels. For thesetests, the slab was bonded with thermally conductive RTV to agallium-filled diode array heat sink assembly 64 having a PCM volume of0.26 in³. Various heat loading levels were used and no damage to theslab was observed at an input power of 55 A, 250 μsec pulsewidth,repetition rate of 500 Hz, and run-time of 20 seconds.

It should be noted that after the system is operated, it returns to itsstarting point prior to operation because the gallium phase changematerial will eventually solidify. Once at its starting point, the lasersystem 10 can be operated again.

To accelerate the solidification of the PCM and reduce the delay beforethe laser diode array assembly can be operated again, the PCM, in analternative embodiment, is in thermal communication with a secondaryheat exchanger utilizing active cooling. This alternative embodiment isgenerally illustrated in FIGS. 10-13 and will now be described.

Referring now to FIG. 10, a laser diode array assembly 210 includes alaser diode array 214, a heat sink assembly 216, and a heat exchanger218. For simplicity, the heat sink assembly 216 and heat exchanger 218will be called a dual-stage temperature control system 220. The heatsink assembly 216 contains a PCM 222 which is in thermal communicationwith an active heat exchanger 218 containing a working fluid 224. Theheat sink assembly 216 includes a base plate 226 and a plurality of fins228. As illustrated, the fins 228 are branched and extend radially fromthe base plate 226 along a length that is preferably as long as thelaser diode array 214. The extended surfaces 230 may have a variety ofshapes and are not limited to the radially branched fins 228 shown inFIG. 10. Other variations include tube fins, spines, grooves, plate finsof other shapes, plate baffle constructions, internal fin-tubeconstructions, and a shell-and-tube construction.

The active heat exchanger 218 includes a contact plate 232 and aplurality of fins 234. The fins 234 extend outwardly from the contactplate 232 preferably along a length at least as long as the laser diodearray 214. The extended surfaces 236 may have a variety of shapes andare not limited to the radially branched fins 234 shown in FIG. 10.Other fin variations for the heat exchanger include tube fins, spines,grooves, plate fins of other shapes, plate baffle constructions,internal fin-tube constructions, and a shell-and-tube construction.

The contact plate 232 conforms closely to the shape generally defined bythe outer perimeter of the fins 228 of the heat sink assembly 216.Before the contact plate 232 is positioned, a retaining plate 238 may beused to enclose the heat sink assembly 216 and generally define a firstchamber 240 in which PCM 222 is placed. Alternatively, without aretaining plate 238, the contact plate 232 alone would serve to enclosethe fins 228 and generally define a first chamber 240 in which the PCM222 is placed. A sheet of indium foil may be laid over and pressed ontoretaining plate 238 to reduce the thermal resistance at the interfacebetween the retaining plate 238 and the contact plate 232.

A second chamber 242 through which the working fluid 224 flows isgenerally defined by a heat exchanger cover 244 that encloses the fins234. The cover 244 and the heat exchanger 218 are firmly secured to thebase plate 226 by passing fasteners 246 into apertures 248 of the baseplate 226 to engage all of the components. The second chamber 242 isprovided with an inlet and outlet for the forced exchange of workingfluid 224. The working fluid 224, which can be any fluid such as air,water, or a fluorocarbon refrigerant, flows through the second chamber242 to receive waste heat from the PCM 222 of heat sink assembly 216.Also, the PCM 222 can be cooled by natural convection of air through theheat exchanger 218. In a further alternative, an expansion bottle,wherein a gas expands from its compressed state, can be used to cool theheat exchanger 218.

FIG. 11 illustrates the laser diode array assembly 210 with analternative dual-stage temperature control system 249 formed byelectrical-discharge machining (EDM). The dual-stage temperature controlsystem 249 includes a first set of cavities 250 for receiving PCM and asecond set of interconnected cavities 252 for receiving working fluid224. Both sets of cavities 250, 252 are formed within the same block ofmetal (e.g. brass or copper) such that the cavities 252 containingworking fluid 224 are interposed between the cavities 250 of PCM.Preferably, the PCM cavities 250 are located proximate to theheat-generating component, such as the diode array 214, relative to thecavities 252 of working fluid. While elongated and radially extendingcavities 250, 252 are depicted in FIG. 11, the cavities 250, 252 may beof any shape, length, and interposed configuration for effective heattransfer.

The laser diode array assembly 210 including yet another embodiment ofthe dual-stage temperature control system 253 formed by EDM is shown inFIG. 12. The dual-stage temperature control system 253 includes a firstset of PCM cavities 254 proximately located to the diode array 214relative to a second set of cavities 256 containing working fluid. Inthis embodiment, all of PCM cavities 254 are adjacent to each anotherand all of the working fluid cavities 256 are adjacent to each other.While elongated and radially extending cavities 254, 256 are shown inFIG. 12, the cavities 254, 256 may be of any shape, length, andconfiguration for effective heat transfer.

While the embodiments shown in FIGS. 10-12 depict dual-stage temperaturecontrol systems 220, 249, 253 used for cooling a diode array 214, thedual-stage temperature control system 220 can be used to cool anyheat-generating component in the laser system. These components includethe laser medium (e.g. ND:YAG), beam dumps, acousto-optic Q-switches,and nonlinear crystals.

With particular reference to FIG. 13, there is shown a laser medium 258in thermal communication with a first dual-stage temperature controlsystem 260 and a diode array 262 in thermal communication with a seconddual-stage temperature control system 264. A fluid circuit 266,schematically illustrated in FIG. 13, is connected to the systems 260,264. The first and second dual-stage temperature control systems 260,264 include respective first stage elements 268, 269 containing PCM andrespective second stage elements 270, 271 utilizing working fluid foractive cooling. The first stage elements 268, 269 of the first andsecond dual-stage temperature control systems 260, 264 are locatedproximate to the laser medium 258 and diode array 262, respectively,relative to the second stage elements 270, 271. The second stageelements 270, 271 have inlets 272, 273, respectively, and outlets 274,275, respectively, for circulating working fluid therethrough andremoving waste heat. The first stage elements 268, 269 and second stageelements 270, 271 can be of any configuration described previously inreference to FIGS. 10-12. Alternatively, the first stage elements 268,269 of the first and second dual-stage temperature control systems 260,264 can be similar to the laser medium heat sink assembly 82 and thediode array heat sink assembly 64, respectively, previously described inreference to FIGS. 4-7.

While each of the second-stage elements 270, 271 can be connected to aseparate fluid circuit, FIG. 13 schematically illustrates a single fluidcircuit 266 having a valve 276, a pump 278, and a heat exchanger 280 foruse with both second-stage elements 270, 271. The fluid circuit 266enables a working fluid, either a liquid or a gas, to be passed througheach second stage element 270, 271 so as to control the temperature ofthe second stage elements 270, 271. This controls the flux of thermalenergy between first and second stage elements 268, 269 and 270, 271,respectively. The temperature of the second stage elements 270, 271 maybe controlled by the circuit 266 by controlling the volumetric flow rateof the fluid through the circuit 266 by the valve 276 or the pump 278,the inlet temperature of the fluid to the second stage elements 270,271, the fin structure, and the physical properties of the fluid.

By controlling the temperature of second stage elements 270, 271, thetemperature of the PCM contained within the first stage elements 268,269 can be maintained at its melt-phase temperature. In turn, theselection of a PCM having a melting temperature approximately equal tothe operating temperature of the laser component, affords proper controlof the temperature of the laser component. Preferably, the meltingtemperature of the PCM is within about 5° C. of the operatingtemperature of the laser component.

Further, if the laser component is to be temporarily operated at ahigher level producing additional waste heat, the system maintains thelaser component at its proper temperature. This is especially usefulwhen the temperature of the working fluid is set at a constanttemperature. In this case, the additional waste heat causes more meltingof the PCM, while still maintaining the temperature of the heat sinkbase plate at approximately the same temperature. Accordingly, the lasercomponent is maintained at the same temperature. Similarly, when thelaser component is temporarily operated at a lower level, producing lesswaste heat, less PCM is melted. Thus, the heat sink with the PCM can bethought of as a thermal buffer allowing for increases and decreases inoperating levels without a change in the temperature of the lasercomponent. In essence, the need for an electronic feedback loop forthermal control of the laser component is avoided as thermal control isprovided by the latent heat of the PCM.

The PCM also provides thermal control of the laser component when thetemperature of the working fluid fluctuates. When the working fluid isthe ambient air and the system is operated without a PCM heat sink, thetemperature of the laser component would generally rise and fall by anamount equal to the change in the ambient temperature. However, a systemhaving a heat sink assembly containing a PCM will better maintain thelaser component at its operating temperature as the temperature of theambient air fluctuates. By way of example, when a heat sink containinggallium is used (i.e. melting temperature of about 30° C.) and theambient air through the heat exchanger is fluctuating between about 20°C. and 30° C., a temperature sensitive laser component in contact withthe heat sink can still be operated at a relatively constant temperature(e.g. about 35° C. to 40° C.).

Referring now to FIG. 14, there is shown a thermoelectric cooler (TEC)282 of the type produced by Marlow Industries, Inc. of Dallas, Tex.disposed between an active heat exchanger 284 and heat sink assembly 286containing PCM 287 in thermal communication with the laser diode array214. The TEC 282 is mounted to the heat sink assembly 286 and the activeheat exchanger 284 by soldering, epoxy, or compression method by the useof fasteners. As shown, the heat sink assembly 286 is firmly secured tothe heat exchanger 284 by passing fasteners 288 into apertures 290 toengage the components. Thus, the heat exchanger 284 receives the heatfrom the heat sink assembly 286 that the thermoelectric cooler 282 pumpsfrom its cool side to its hot side, as well as the waste heat from thethermoelectric cooler 282 itself The heat exchanger 284 then releasesthis heat to a working fluid flowing therethrough.

The TEC 282 is a solid state heat pump that operates on the Peltiertheory. A typical TEC 282 consists of an array of semiconductor elements292 that act as two dissimilar conductors that create a temperaturedifference when a voltage is applied to their free ends. The array ofsemiconductor elements 292 is soldered between two ceramic plates 294,electrically in series and thermally in parallel. As a current passesthrough the elements, there is a decrease in temperature at the coldside 296 resulting in the absorption of heat from the environment. Theheat is carried through the cooler by electron transport and released onthe opposite side 298 as electrons move from a high to low energy state.To cool the TEC 282, the active heat exchanger 284 is disposed adjacentto the “hot side” 298 of the TEC 282 to carry away the discharged heat.

The TEC 282, which is in thermal communication with the heat sinkassembly 286, can serve to draw heat from the heat sink assembly 286 andsolidify the liquid form of PCM 287 so that the laser diode arrayassembly 210 can be operated without much delay and overheating. Forexample, this embodiment is especially useful in situations where theambient temperature of the laser diode array assembly 210 is greaterthan the melting temperature of the PCM 287. The TEC 282 cooling theheat sink assembly 286 will solidify the PCM 287 so as to keep the lasercomponent from overheating. Also, with the reversal of the currentpassing through the TEC 282, the TEC 282 can serve to raise thetemperature of the PCM 287 for the thermal control of other systemcomponents requiring raised temperatures.

With particular reference now to FIG. 15, there is shown anotherembodiment of the present invention wherein the TEC 282 is disposedbetween the laser diode array 214 and a PCM-filled heat sink assembly286. The TEC 282, which is in thermal communication with the heat sinkassembly 286 and laser diode array 214 or other heat-generating systemcomponent, is mounted to the laser diode array 214 and heat sinkassembly 286 by soldering, epoxy, or compression method by the use offasteners. In this embodiment, the heat emitted by the laser diode array214 or other heat-generating component passes through the TEC 282 and isdischarged into the PCM-filled heat sink assembly 286. Once the coolingrequirements of the system component are defined and the maximum heatload to be transferred by the TEC 282 calculated, the proper PCM 299with the appropriate phase change temperature can be selected toefficiently operate the system without undue thermal strain on any ofthe components.

As mentioned above, nonlinear optical (NLO) crystal assemblies for theconversion of a first wavelength into a second wavelength typicallyutilize temperature control systems for the precise control of thesetemperature-sensitive crystals. An embodiment for the thermal control ofNLO crystals 300 such as potassium titanyl phosphate (KTP) and lithiumtriborate is shown in FIG. 16. A PCM-filled heat sink assembly 302 isdisposed between a heater 304 and the NLO crystal 300 which is mountedto the optics bed 12 with fasteners 306. The NLO crystal 300 ismaintained at an ideal temperature by the heat transfer from theadjacent heat sink assembly 302 filled with a PCM 307 having a phasechange temperature generally coincident with the crystal's idealtemperature (e.g. within 5° C. or less). The heat sink assembly 302 isheated by the heater 304 to keep the PCM 307 in its melt phase so thatthe NLO crystal 300 in intimate contact with the PCM 307 will be held ata generally constant temperature that coincides with the meltingtemperature of the PCM 307.

Referring now to FIG. 17, there is shown a laser diode array 214 inthermal communication with a heat sink assembly 308 having a pluralityof cavities 310 filled with two-types of PCM 312, 314 each having twodifferent melting temperatures. Preferably, a PCM 312 having a highermelting temperature is contained in cavities 310 closer to theheat-generating device relative to the cavities 310 filled with a PCM314 having a lower melting temperature. The PCM-filled cavities 310proximate to the heat-generating component serve to passively cool itwhile those further away from the heat-generating component serve as asecondary heat sink for the system. The low-temperature PCM 314 isselected to maintain the high-temperature PCM 312 generally in its meltphase based on the heat load of the laser diode array. While twodistinct sets of PCM-filled and EDM-formed cavities 310 are shown inFIG. 17, more than two-types of PCM can be used to tailor thetemperature gradient along the length and width of the heat-generatingcomponent. Furthermore, EDM and non-EDM cavities of various shapes,sizes, and configurations are also possible. This dual PCM configurationcan be used in the heat sinks of the systems described above.

Each of these embodiments and obvious variations thereof is contemplatedas falling within the spirit and scope of the invention, which is setforth in the following claims.

What is claimed is:
 1. A laser system, comprising: a thermal controlsystem for producing passive cooling of a laser temperature-sensitivecomponent that produces heat, said temperature-sensitive componenthaving a desired operating temperature range, said thermal controlsystem comprising; a heat sink assembly in thermal communication withsaid temperature-sensitive component to remove heat therefrom, said heatsink assembly containing a substantially solid form of a phase changematerial, wherein said removed heat causes said phase change material tochange from said solid form to a liquid form at a predetermined phasechange temperature; and a heat exchanger in thermal communication withsaid heat sink assembly to remove heat from said heat sink assembly soas to maintain said phase change material at about said phase changetemperature, said heat exchanger transferring heat from said heat sinkassembly to a working fluid in thermal communication with said heatexchanger to thereby maintain said temperature-sensitive componentwithin said desired operating temperature range and thereby enhance theperformance of said temperature-sensitive component.
 2. The laser systemof claim 1, wherein said phase change material is one material selectedfrom the group consisting of gallium, alkyl hydrocarbons, salt hydrates,and low temperature metallic alloys.
 3. The laser system of claim 1,wherein said laser temperature-sensitive component is one of thecomponents consisting of an acousto-optic q-switch, a nonlinear crystal,a beam dump, a laser diode array, and a laser medium.
 4. The lasersystem of claim 1, wherein said heat exchanger is directly attached tosaid heat sink assembly.
 5. The laser system of claim 1, wherein saidheat sink assembly includes fins having upper and lower portions anddefining gaps therebetween, said phase change material being placed insaid gaps adjacent to said upper portion, said lower portion of saidfins being a part of said heat exchanger.
 6. The laser system of claim1, wherein said phase change material is gallium and said working fluidis maintained below about 30° C.
 7. The laser system of claim 1, whereinsaid temperature-sensitive component has an operating temperature, saidphase change temperature being approximately equal to a desiredoperating temperature.
 8. The laser system of claim 1, wherein saidtemperature-sensitive component has an operating temperature, said phasechange temperature being within about 5° C. of a desired operatingtemperature.
 9. The laser system of claim 1, wherein said heat exchangeris integrally formed with said heat sink assembly.
 10. The laser systemof claim 9, wherein said heat exchanger has fins which are interposedwith fins of said heat sink assembly.
 11. The laser system of claim 1,wherein said heat sink assembly defines a plurality ofphase-change-material cavities for receiving said phase change material,said heat exchanger defines a plurality of working fluid cavities forreceiving said working fluid.
 12. The laser system of claim 11, whereinsaid phase-change-material cavities are proximately located to saidtemperature-sensitive component relative to said working fluid cavities.13. The laser system of claim 11, wherein said phase-change-materialcavities are interposed with said working fluid cavities.
 14. The lasersystem of claim 1, further including a fluid circuit, connected to saidheat exchanger, said fluid circuit providing movement to said workingfluid for forced convection heat transfer.
 15. The laser system of claim1, wherein said heat exchanger is capable of receiving heat from saidworking fluid and transfers heat to said heat sink assembly in thermalcommunication with said heat exchanger.
 16. The laser system of claim 1,wherein said heat sink assembly includes a base plate with a pluralityof surfaces extending from said base plate and forming intersticestherebetween, said phase change material being in contact with saidextended surfaces, said plurality of surfaces extending radially fromsaid base plate.
 17. The laser system of claim 1, wherein said workingfluid is air.
 18. The laser system of claim 1, wherein said heatexchanger continues to remove heat from said heat sink assembly afteroperation of said temperature-sensitive component to cause phase changematerial to return to said solid form.
 19. A laser system, comprising: athermal control system for producing passive cooling of a lasertemperature-sensitive component that produces heat, saidtemperature-sensitive component having a desired operating temperaturerange, said thermal control system comprising; a heat sink assembly inthermal communication with said temperature-sensitive component toremove said heat therefrom, said heat sink assembly containing asubstantially solid form of a phase change material, and wherein saidremoved heat causes said phase change material to change from said solidform to a liquid form at a predetermined phase change temperature; and aheat exchanger in thermal communication with said heat sink assembly,said heat exchanger transferring heat from said heat sink assembly toair that is in thermal communication with said heat exchanger formaintaining said phase change material at about said phase changetemperature and thereby to enhance the performance of saidtemperature-sensitive component by maintaining saidtemperature-sensitive component within said desired operatingtemperature range.
 20. The laser system of claim 19, wherein said air isforced through said heat exchanger.
 21. The laser system of claim 19,wherein said air is passing through said heat exchanger via naturalconvection.
 22. The laser system of claim 19, wherein said heatexchanger continues to remove heat from said heat sink assembly afteroperation of said temperature-sensitive component to cause said phasechange material to return to said solid form.